1. Field of the Invention
The present invention relates to a depolarizing film adapted to be disposed on a polarizing plate of a kind used on a surface of a liquid crystal display device, an organic electroluminescent display device or the like and, also, to a method of making such depolarizing film. More specifically, the present invention relates to the film of the kind referred to above, which is effective to avert a possible reduction in visibility that may lead to darkening of the display when the latter is viewed with polarizing eyeglasses.
2. Description of Related Art
The liquid display device generally includes a liquid crystal cell, made up of two transparent substrates with a liquid crystal composition filled in a gap between those substrates, and one set of polarizing plates each disposed on one of opposite sides of the liquid crystal cell. In the liquid crystal display device of the structure discussed above, when an electric voltage is applied to the liquid crystal composition through transparent electrodes deposited on the inner surfaces of the transparent substrates, the orientation of the liquid crystal composition changes to permit light from a backlighting source to pass through or be intercepted by the liquid crystal cell.
The liquid crystal display device is generally classified into TN type, STN type, VA type, IPS type, OCB type and so on depending on the light modulating system of liquid crystal molecules used. However, regardless of the specific type, the liquid crystal display device in general makes use of a set of polarizing plates together with the liquid crystal cell. Accordingly, in this type of the liquid crystal display device utilizing the polarizing plates, a viewer perceives a portion of the incoming light which has passed through the polarizing plates disposed on the liquid crystal cell and has hence been linearly polarized.
On the other hand, the organic electroluminescent display device is known to be of a type in which a layer of electroluminescent material accommodated between two layers of metal conductors emits visible light when an electric current is applied thereto, but the metal conductors have such a high light reflectivity that the incoming external light other than the visible light generated from the electroluminescent layer may be reflected by such metal conductors enough to reduce in contrast of the image being displayed. In order to avoid the reflection of the external light upon the metal conductors, circular polarizing plates each made up of a ¼ wavelength plate and a polarizing plate are employed and the front most surface of the display device is covered with a linear polarizing plate. Accordingly, even with the organic electroluminescent display device, a viewer perceives a portion of the incoming light which has passed through the polarizing plate on the surface of the display device and has hence been linearly polarized.
In recent years, with the wide spreading use of the liquid crystal display device or the organic electroluminescent display device on the occasion of enjoying audio-visual presentations of one-segment terrestrial digital audio/video and data broadcasting services, and in automobile instrument panels and car navigation systems, which is promulgated by countrywide adaption of the terrestrial digital television broadcasting system developed for portable appliances such as, for example, mobile cells to receive it, the opportunity is increasing, in which the liquid crystal display device or the organic electroluminescent display device is used in the outdoor environment or at a site where the external light is intensive.
On the other hand, the polarizing eyeglasses are effective to remove a reduction in visibility, which would otherwise result from reflection of an automobile dashboard on the front windshield during the driving of an automotive vehicle, to remove a glaring caused by reflection of the sun light on a rear windowpane of the leading car, to reduce glaring on a water surface during fishing or marine sports and so on. For this reason, it is quite often that a viewer wears the polarizing eyeglasses when looking at a liquid crystal display device or an organic electroluminescent display device. When the viewer wears the polarizing eyeglasses when looking at the liquid crystal display device or the organic electroluminescent display device, the displayed image will be darkened when the polarizing absorption axes of the polarizing eyeglasses and the polarizing axis of light emerging from the liquid crystal display device match with each other, thus posing a problem associated with a considerable reduction in visibility.
The JP Laid-open Patent Publication No. H10-10522, published in the year of 1998, suggests the provision of a depolarizing means on the liquid display surface to thereby alleviate the problems and inconveniences discussed hereinabove. According to this prior art publication, the depolarizing means is in the form of a depolarizing plate comprised of a combination of two quartz plates. It is, however, been found that covering of an entire display screen of the liquid crystal display device with the depolarizing means in the form of the combination of the two quartz plates is not practical. Also, the JP Laid-open Patent Publication No. H10-10523, also published in the year of 1998, discloses the use of a double refraction plate such as, for example, a ¼ wavelength plate for converting the display light, emerging outwardly from the liquid crystal display surface, from a linearly polarized light into a circular polarized light or an elliptically polarized light. Although with this method according to the second mentioned prior art publication, there is no possibility that the darkening occurs even when the polarizing eyeglasses are worn on, a problem arises that the color tone of the displayed image may be shaded depending on the direction in which the head is tilted leftward or rightward.